This invention relates to an improvement to my Low-Temperature Engine system (LTES) described in U.S. Pat. No. 4,503,682, which is incorporated herein by reference, and other turbine systems employing organic Rankine cycles (ORC).
The use of hydrocarbon media in organic Rankine power turbine cycles has been a developing art for some time. Many of these media possess the property of a characteristic saturation curve with a reverse slope to that of their isentropic curves across the range of pressures and temperatures traversed during operation of the turbine cycle between an elevated temperature and pressure condition at the turbine entry to the intended turbine exhaust pressure. When such media have acquired some level of superheat during that isentropic expansion process, the superheat level becomes an additional amount of waste heat that must be rejected during the condensation process before liquefying of the vapor at the saturation temperature for that exhaust pressure can begin. Normal butane, isobutane, and isopentane are among the more common hydrocarbon turbine media exhibiting this characteristic as well as several of the fluorinated hydrocarbons that have been employed as turbine media.
Numerous efforts to minimize this source of waste heat loss have been reported in the literature. Among them have been:
a. Efforts to introduce the medium at the turbine entry in a "wet vapor" condition in an effort to permit the vapor to dry during its expansion cycle through the turbine to arrive at the turbine exit in a saturated or less superheated condition. This approach has unfortunately encumbered a problem due to piping effects between the hydrocarbon boiler and the turbine resulting in separation of some of the moisture content into "slugs" of liquid which, upon entering the turbine, cause damage or erosion to the turbine blading. PA1 b. The use of a supercritical entry condition under which the medium, at a supercritical temperature and pressure, enters the turbine and expands through regions in which it passes through wet vapor conditions to reemerge dryer as expansion continues, ultimately arriving at the turbine exit pressure at, or very nearly at, its saturation temperature for that pressure. This method unfortunately is limited to applications in which the peak temperature available in the boiler is sufficient to produce a temperature in the supercritical region, and often requires the expenditure of excessive parasitic plant power to pump the turbine medium up to whatever pressure may be required to reach the supercritical pressure necessary for the turbine medium employed. PA1 c. The use of a "recuperator" has frequently been introduced. This item consists of a heat exchanger placed between the turbine exhaust vapor and the returning turbine medium condensate from the condenser. This permits most of the superheat to be regeneratively recovered by the returning liquid turbine medium feed stream. The unrecoverable superheat is reduced to that involved to maintain the minimum approach difference in temperature across the recuperator heat exchanger.
The LTES cycle achieves the output power increase it offers by virtue of an extended turbine cycle operating between the temperature of the ambient temperature condenser of a conventional turbine cycle and a sub-ambient condenser made available by artificial refrigeration. In order to accomplish this without expending more external heat energy to operate the refrigeration sub-system than that which could have produced more output power by being supplied to a conventional turbine cycle, an absorption refrigeration subsystem (AR system) is employed whose above ambient waste heat rejection is recovered regeneratively by turbine medium feed stream heating.
However, the contribution toward producing an output power increase offered by the LTES is directly proportional to the ratio of the mass flow in the above ambient portion of the turbine cycle to that able to be condensed by the refrigeration capacity of the AR subsystem. That contribution to the total system output is diluted by the extent to which enough of the above-ambient turbine medium mass flow must be supplied which has the capacity to absorb the latent heat rejected from the AR subsystem to condense the refrigerant circulating therethrough. The temperature gradient across which this heat rejection must occur is limited to the temperature differential between the ambient turbine condenser and the saturation temperature of the AR subsystem refrigerant at its condenser pressure.
Because of that situation, raising the temperature of the ambient turbine condensate above its condensation temperature by use of a conventional recuperator reduces the remaining thermal gradient available to absorb additional heat rejected from the AR subsystem condenser, with a consequent increase of the total above-ambient mass flow in the turbine cycle being needed to provide the remaining heat absorption capacity. Such a situation further dilutes or negates the advantage of the LTES concept.